U.S. patent application number 12/490484 was filed with the patent office on 2013-10-03 for sensors incorporating freestanding carbon nanostructures.
This patent application is currently assigned to LEHIGH UNIVERSITY. The applicant listed for this patent is Jiri Cech, Himanshu Jain, Venkataraman Swaminathan. Invention is credited to Jiri Cech, Himanshu Jain, Venkataraman Swaminathan.
Application Number | 20130256627 12/490484 |
Document ID | / |
Family ID | 49233654 |
Filed Date | 2013-10-03 |
United States Patent
Application |
20130256627 |
Kind Code |
A1 |
Jain; Himanshu ; et
al. |
October 3, 2013 |
Sensors Incorporating Freestanding Carbon NanoStructures
Abstract
Sensors for detecting IR radiation, UV radiation, X-Rays, light,
gas, and chemicals. The sensors herein incorporate freestanding
carbon nanostructures, such as single-walled carbon nanotubes
("SWCNT"), atomically thin carbon sheets having a thickness of
about between 1 atom and about 5 atoms ("graphene"), and
combinations thereof. The freestanding carbon nanostructures are
suspended above a substrate by a plurality of conductors, each
conductor electrically connected to the carbon nanostructure. In
one method of manufacture, a resonance chamber is formed under the
carbon nanostructure by etching of the substrate, yielding a sensor
wherein the resonance chamber is bounded by at least the substrate
and the carbon nanostructure.
Inventors: |
Jain; Himanshu; (Bethlehem,
PA) ; Swaminathan; Venkataraman; (Bridgewater,
NJ) ; Cech; Jiri; (Bethlehem, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Jain; Himanshu
Swaminathan; Venkataraman
Cech; Jiri |
Bethlehem
Bridgewater
Bethlehem |
PA
NJ
PA |
US
US
US |
|
|
Assignee: |
LEHIGH UNIVERSITY
Bethlehem
PA
|
Family ID: |
49233654 |
Appl. No.: |
12/490484 |
Filed: |
June 24, 2009 |
Current U.S.
Class: |
257/9 ;
438/57 |
Current CPC
Class: |
H01L 27/305 20130101;
B82Y 10/00 20130101; G01J 5/023 20130101; H01L 51/0048 20130101;
G01J 5/20 20130101; G01J 5/024 20130101; G01J 5/0853 20130101; H01L
31/028 20130101; H01L 31/1804 20130101 |
Class at
Publication: |
257/9 ;
438/57 |
International
Class: |
H01L 31/028 20060101
H01L031/028; H01L 31/18 20060101 H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0001] This invention was made with government support under
Contract No. W911NF-07-2-0064 respectively awarded by the
Department of Defense--U.S. Army Research Lab (ARL). The government
may have certain rights in the invention.
Claims
1. A sensor comprising; a substrate; a freestanding nanocarbon
structure suspended between a plurality of conductors, each
conductor electrically connected to the nanocarbon structure; and a
resonance chamber, wherein the resonance chamber is bounded by at
least the substrate and the freestanding nanocarbon structure.
2. The sensor of claim 1, wherein the nanocarbon structure
comprises at least one of single-walled carbon nanotubes and
graphene.
3. The sensor of claim 2, wherein the substrate comprises an
intermediate sacrificial layer and a base layer, wherein the
intermediate sacrificial layer is located between the nanocarbon
structure and the base layer of the substrate.
4. The sensor of claim 2 wherein the depth of the resonance chamber
is selected in relation to a radiation wavelength (.lamda.).
5. The sensor of claim 2 wherein at least a portion of the
nanocarbon structure is separated from the substrate layer by the
resonance chamber.
6. The sensor of claim 2 wherein the resonance chamber boundaries
comprise the nanocarbon structure and the substrate.
7. The sensor of claim 6 wherein the chamber boundaries further
comprise at least one of the conductors, the intermediate
sacrificial layer; or the substrate.
8. The sensor of claim 2 wherein the base substrate comprises
materials suitable for substrate use in lithographic processes.
9. The sensor of claim 8, wherein the material suitable for
substrate use in lithographic processes comprises Si.
10. The sensor of claim 9, wherein the intermediate sacrificial
substrate comprises at least one oxide of Si.
11. A method of manufacturing the sensor of claim 1, the method
comprising the steps of: a) providing a substrate; b) generating a
nanocarbon structure on at least one selected exposed surface of
the substrate; c) connecting the nanocarbon structure to at least
two conductors; and d) forming the resonance chamber by
underetching at least a portion of the substrate surface underlying
the carbon structure.
12. The method of claim 11, wherein the carbon nano structure
comprises at least one of single-walled carbon nanotubes and
graphene.
13. The method of claim 12, wherein the substrate comprises an
intermediate sacrificial layer and a base layer, wherein the
intermediate layer is located between the carbon nanostructure and
the base layer of the substrate.
14. The method of claim 12 wherein the depth of the resonance
chamber is selected in relation to a radiation wavelength
(.lamda.).
15. The method of claim 12 wherein at least a portion of the carbon
nanostructure is separated from the substrate layer by the
resonance chamber.
16. The method of claim 12 wherein the resonance chamber boundaries
comprise the carbon nanostructure and the substrate.
17. The method of claim 16 wherein the chamber boundaries further
comprise at least one of the conductors, the intermediate
sacrificial layer; or the substrate.
18. The method of claim 17 wherein the network of nanotubes extends
between the plurality of conductors.
19. The method of claim 12 wherein the base substrate comprises
materials suitable for substrate use in lithographic processes.
20. The method of claim 19, wherein the material suitable for
substrate use in lithographic processes comprises Si.
21. The method of claim 20, wherein the intermediate sacrificial
substrate comprises at least one oxide of Si.
22. A method of manufacturing the sensor of claim 1, comprising the
steps of (a) providing a substrate comprising a multi-layered
Si/SiO.sub.2 chip; (b) generating a network of nanotubes on a
selected exposed surface of the substrate using chemical vapor
deposition (CVD); (c) providing conductors on the selected exposed
surface of the substrate; and (d) removing at least a portion of
the previously exposed selected substrate surface underlying the
nanotube network to form a resonance chamber to yield a
freestanding nanotube network that spans between at least two
conductors to form a boundary of the resonance chamber; wherein the
remainder of the chamber is bounded by any of the substrate, the
conductors, and combinations thereof.
Description
FIELD OF INVENTION
[0002] Sensors for detecting IR radiation, UV radiation, X-Rays,
light, gas, and chemicals. The novel sensors herein incorporate
freestanding carbon nanostructures, such as single-walled carbon
nanotubes ("SWCNT"), atomically thin carbon sheets (including those
having a thickness of about between 1 atom and about 5 atoms
commonly known as "graphene"), and combinations thereof.
BACKGROUND
[0003] There is a pressing need for new sensors of low energy, low
intensity radiation. The advanced cameras, sensors and imaging
techniques find applications not only in astronomy, but also in
security, military, medical and biological applications. With a
continuing need to detect radiation with wavelength as high as 100
.mu.m, there exists the need for sensitive bolometers with fast and
accurate response.
[0004] Recently, extremely large photoresponse was reported for
suspended single-walled carbon nanotube (SWCNT) films (See H.
Jerominek, F. Picard, D. Vincent, Vanadium oxide films for optical
switching and detection, Optical Engineering 32 (1993) 2092-2099)
which makes them an attractive candidate for the sensitive element
of an infrared (IR) bolometer and microbolometer focal plane
arrays. Optical properties of SWCNT networks, including
photoconductivity, suggest outstanding potential for application in
nanoscale optoelectronics. See H. Jerominek, T. D. Pope, M. Renaud,
N. R. Swart, F. Picard, M. Lehoux, S. Savard, G. Bilodeau, D.
Audet, L. N. Phong, C. N. Qiu, 64 64, 128 128 and 240 320 pixel
uncooled IR bolometric sensor arrays, Proceedings of SPIE 3061
(1997) 236-247. For example, conventional micro-bolometer with thin
film absorber, fabricated by e-beam lithography. See M. Moreno et
al., J. Non-Cryst. Solids (2008),
doi:10.1016/j.jnoncrysol.2007.09.116 (in press) has a volume of
10.times.0.2.times.0.05=0.1 .mu.m.sup.3. By replacing metal with
carbon nanotube network, one should be able to reduce this volume
by 3-4 orders. See A. Naemee et al. ACM Proc., San Diego, 568-573
(2007).
[0005] The sensitivity of detection among different living species
that rely on detecting heat/IR radiation, such as the fire-seeking
beetles (Melanophila acuminate), has been estimated to be
approximately 60 .mu.W/cm.sup.2. See M. E. Itkis, et al, Science 21
Apr. 2006: Vol. 312. no. 5772, pp. 413-416. That is significantly
better than the IR sensors available today. Thin films of SWCNT
have shown bolometric response time as short as 50 ms. The
extremely fast bolometric response of a freestanding network is
expected to be 5-10 orders more sensitive to IR intensity than the
previously reported photoresponse of network on substrates, where
the main limitation is ultrafast relaxation time of photocarriers
(10.sup.-10 to 10.sup.-14 s). See, P. V. Avramov, P. B. Sorokin, A.
S. Fedorov, D. Fedorov, Y. Maeda, Phys. Rev. B 74, 245417 (2006).
For the latter, bolometric response is obviously limited by thermal
coupling to substrate. In this regard, note that the prominent
features in the optical spectra of SWCNT have been widely
attributed to inter-band transitions associated with series of van
Hove singularities in the one-dimensional (1D) density of states.
See U. Dettlaff, V. Skakalova, J. C. Meyer, J. Cech, B. Mueller,
and S. Roth. Effect of fluorination on electrical properties of
single walled carbon nanotubes and C peapods in networks. Current
Applied Physics 7, 42-46 (2007). However, recent studies suggest
that the electron-hole pairs are strongly coupled in 1D lattice and
that the major photoexcitations are excitons, rather than free
carriers. Id. H. Jerominek 2092-2099, and H. Jerominek 236-247. The
response of a CNT network for bolometric applications does not
appear to be by free carriers photoconductivity for the following
reasons: (i) The response is strongly decreased by thermal coupling
to substrate or to environment, (ii) The time constant is typically
1-100 ms and (iii) Magnitude of bolometric response depends on the
temperature derivative of resistance dR/dT. Finally, the absorption
coefficient .alpha. of SWCNT is extremely high, (10.sup.3 to
10.sup.4 cm.sup.-1) i.e. at least an order of magnitude greater
that for HgCdTe. It also extends far to the low energy region,
without much of a drop in performance. An absolute value of
temperature coefficient of resistance (TCR) is at least as high as
observed with conventional vanadium dioxide films.
BRIEF STATEMENT
[0006] A sensor is provided for detecting IR radiation, UV
radiation, X-Rays, light, gas, and chemicals. The sensors include:
a substrate; a freestanding carbon nanostructure suspended between
a plurality of conductors, each conductor electrically connected to
the carbon nanostructure; and a resonance chamber, wherein the
resonance chamber is bounded by at least the substrate and the
freestanding carbon nanostructure.
[0007] A method is provided for manufacturing a sensor
incorporating a freestanding carbon nanostructure. In one
embodiment, the method includes the steps of: (a) providing a
substrate comprising a multi-layered Si/SiO.sub.2 chip; (b)
generating a network of nanotubes on a selected exposed surface of
the substrate using chemical vapor deposition (CVD); (c) providing
conductors on the selected exposed surface of the substrate; and
(d) removing at least a portion of the previously exposed selected
substrate surface underlying the nanotube network to form a
resonance chamber to yield a freestanding nanotube network that
spans between at least two conductors to form a boundary of the
resonance chamber; wherein the remainder of the chamber is bounded
by any of the substrate, the conductors, and combinations thereof.
Additional features may be understood by referring to the
accompanying drawings, which should be read in conjunction with the
following detailed description and examples
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a schematic drawing of an exemplary sensors and
its method of fabrication in accordance with one embodiment
herein.
[0009] FIG. 2 shows a Scanning Electron Microscope (SEM) image of a
substantially uniform network of single-walled carbon nanotubes
that is representative of a nanotube network in one embodiment
herein.
[0010] FIG. 3 shows an elevational view of an exemplary sensor in
accordance with another embodiment herein.
[0011] FIG. 4 shows an elevational view of exemplary sensor in
accordance with yet another embodiment herein.
DETAILED DESCRIPTION OF THE DRAWINGS
[0012] The novel sensors herein incorporate freestanding carbon
nanostructures, such as single-walled carbon nanotubes ("SWCNT"),
atomically thin carbon sheets (including those commonly known as
graphene sheets and having a thickness of about between 1 atom and
about 5 atoms ("graphene")), and combinations thereof. For example,
nanotube networks, and particularly SWCNT networks, can be a
sensitive sensor of low energy radiation. Indeed, the inventors
have conceived that a practical viability of such a nanotube
network exists, both in terms of structure of an apparatus and
methods of fabrication. The inventors have conceived of methods to
build and operate novel sensors incorporating at least one thin
suspended yet freestanding (relevant to a substrate) carbon
nanostructure, such as a network of nanotubes, and/or a graphene
sheet. In one embodiment, the carbon nanostructure includes a
sparse, substantially uniform network of single walled carbon
nanotubes ("SWCNT"). Additionally, the inventors have developed an
understanding of the underlying mechanisms of the electrical
response of a suspended SWCNT network to IR radiation and
temperature.
[0013] A first embodiment and design of the proposed sensor is
shown in FIG. 1. That embodiment is based on established micro
electromechanical system (MEMS) technology. For example, as shown
in FIG. 1, the sensor 10 can be fabricated by employing a
multiple-step fabrication process. In the embodiment shown in FIG.
1, the process involves the labeled steps of: (a) providing a
substrate 12, such as a multi-layered Si/SiO.sub.2 chip (base layer
14 comprising Si, and sacrificial layer 16 comprising an oxide of
Si such as SiO.sub.2) with spin coated Ni/Co compound catalyst; (b)
generating a carbon nanostructure 20 such as a network of nanotubes
on a selected exposed surface of the substrate 12, such as a SWCNT
network grown using chemical vapor deposition (CVD); (c) providing
conductors 30 on the exposed surface of the substrate 12, such as
lithographically deposited electrical contacts in electrical
contact with the network of nanotubes; and (d) removing at least a
portion of the previously exposed selected substrate surface
underlying the nanotube network to form a resonance chamber 40,
such as by chemically underetching a portion of the substrate 12
(such as a portion of sacrificial layer 16) to yield a freestanding
nanotube network that spans between at least two conductors 30 to
form a boundary of the resonance chamber 40, the remainder of the
chamber 40 being bounded by any of the substrate 12 (layers 14,
16), the conductors 30, and combinations thereof. The resonance
chamber 40 enhances response, such as bolometric response, of the
apparatus.
[0014] In a further embodiment, the method of fabricating the
sensor 10 is provided as follows.
[0015] (a) The first step is the preparation of a sparse, CVD grown
network of clean single walled nanotubes on a selected substrate
12, preferably a substrate 12 having a base layer 14 that is
selected to be resistant to etching based on a selected etchant,
such as HF. For example, the base layer may include Si, which is
resistant to etching by HF. The substrate 12 further includes a
sacrificial layer 16 overlying the base layer, the sacrificial
layer selected to be susceptible to etching based on the selected
etchant. For example, the sacrificial layer 16 may include
SiO.sub.2, which is susceptible to etchant HF. As generally
described in references in J. A. Robinson, E. S. Snow, S. C.
Badescu, T. L. Reinecke, and F. K. Perkisn. Role of defects in
single-walled carbon nanotube chemical sensors. NANO LETTERS 6,
1747-1751 (2006), Infrared Sensor and Systems, E. L. Dereniak and
G. D. Boreman, Wiley, New York (1996) p. 414., F. Niklaus, C.
Jansson, A. Decharat, J. Kalhammer, H. Pettersson, and G. Stemme,
Proc. SPIE 6542, 65421M (2007), which are hereby incorporated by
reference, carrier gas (such as an argon/hydrogen mixture) is
passed with vapor phase carbon feedstock through heated quartz tube
placed in a moving tube furnace. A catalyst is provided, such as a
spin-coated or evaporated organometallic compound or salt, or a
plurality of such compounds. The carrier gas mixture is introduced
to the bubbler with carbon feedstock. While the inventors conceive
that a well-tuned carrier-feedstock-catalyst system can be operated
even at atmospheric pressure, a reduced atmosphere will also
produce desirable results. For example, using Ni/Co based catalyst
deposited by spin coating, with ethanol as carbon source will
provide a substantially uniform network of SWCNTs, as
characteristically shown in FIG. 2, which is an SEM micrograph of a
CVD grown nanotube network on chip (field enhancement of SWCNTs,
actual diameter of tubes is much smaller, such as about 1-2 nm).
Id. J. A. Robinson, 1747-1751. This process is useful not only for
providing an individual bolometer by deposition on a single chip,
but also for manufacturing of arrays having virtually identical
microbolometers. Characterization of the SWCNT networks is
accomplished by scanning electron microscope (SEM) with in-lens
secondary electrons (SE) sensor.
[0016] (b) After synthesis of a carbon nanostructure 20 such as a
SWCNT network or graphene sheet on a selected substrate 12 as
described in step a above, such as silicon base substrate 14 with a
SiO.sub.2 sacrificial layer 16, conductors 30 such as gold or
gold-palladium alloy readout contacts are provided. The conductors
30 can be provided by known methods such as conventional or e-beam
lithography. In one example, conductors 30 are provided using
spin-coated PMMA as a photoresist and sputtering/evaporation as
contact deposition technique. It is desirable to next characterize
electrical parameters of contacted networks, while still on the
substrate 12, and before the next fabrication step, to verify the
desired performance of the carbon nanostructure 20 in concert with
the conductors 30 connected to the nanostructure 30.
[0017] (c) The next fabrication step involves underetching the
resonator (also referred to as a cavity or resonance chamber 40
herein) under the carbon nanostructure 20. For example, that will
yield a freely suspended carbon nanostructure 20, such as nanotube
network. For example, as shown in FIG. 3, underetched, freestanding
nanotubes are provided wherein metal contacts are approx. 300 nm
wide. Ideally, such chamber 40 should have depth of 1/4 of design
wavelength (.lamda.). For example, acceptable chambers 40 can be
formed by using dilute HF as the etchant in combination with a
sacrificial layer 16 comprising SiO.sub.2. Where a deeper cavity is
desired that extends through the sacrificial layer 16 and into the
base substrate 14, warm KOH is selected as an etchant for a base
substrate 14 comprising Si. Both etchants are compatible with
conductors 30 (such as gold and other electrically conductive
materials) and in particular with SWCNT networks. The etchant(s)
should be selected so as not to undesirably alter the SWCNT or
other carbon nanostructure 20 network, such as by undesirable
functionalization and/or chemical damage.
[0018] (d) Upon completion of etching step c above, such as while
the apparatus 10 is still in the etchant liquid, a drying step is
performed. For example, a supercritical drying step is desirable in
order to avoid damage to the subtle MEMS (micro-electrical
mechanical system) structure, such as by undesirable surface
tension of a moving liquid-vapor interface.
[0019] Upon completion of the above steps, the apparatus 10 is
characterized by the presence of freestanding carbon nanostructures
20 suspended over a resonance chamber 40. The depth of the chamber
40 is controllable by selection of etchant(s) and etching methods
and times, and can be monitored by scanning electron microscope
(SEM) for example, to check shape and depth of prepared chamber 40.
For example, the next step may involve making contacts between the
conductors 30 and other electrical leads on the chip carrier, and
the bonding pads. Alternatively, the flip-chip process for making
electrical contacts can be used.
[0020] Further, evaluation of the fabrication process and the
resulting apparatus 1, such as bolometers, can be evaluated. For
example, one can measure electro-optical response of prepared
devices, employing lock-in amplifier to record the amplitude of the
voltage oscillation on the load resistance in a standard dc bias
circuit. The load resistance should be maintained at or close to
the value of SWCNT sample for maximum detection efficiency. By way
of further example, IR radiation will be modulated by chopper to
provide desired levels for detection.
[0021] One goal is to provide an IR sensor that meets or exceeds
bolometers that are known in nature and in science. For example,
the sensitivity of detection among different living species that
rely on detecting heat/IR radiation, such as the fire-seeking
beetles (Melanophila acuminate), has been estimated to be
approximately 60 .mu.W/cm.sup.2. The "average" SWCNT (assuming the
diameter of (10,10), (8,8) and (12,0) tubes) has 136 carbon atoms
per nm. Assuming 50 um wide contacts, with .about.10 nanotubes per
1 um, we expect 500 tubes per contact. We multiply it by the length
of the gap (in nm), say 30 um, to give 2.04e9 carbon atoms, which
would weigh 4.06-14 grams. Assuming worst case scenario, the heat
capacity could be 3k.sub.B/(mass of C atom in gm) .about.2078
mJ/gK, we obtain heat capacity of 8.4e-14 J/K for the whole
network. However, if we use experimentally reported heat capacity
for graphite at room temperature (710 mJ/gK), the heat capacity of
our sample network would be 2.88e-14 J. For MWCNTs C .about.720
mJ/gK is found, which is nearly identical to the value for
graphite. Note that heat capacity of MWCNTs agrees with that of
graphite above the crossover temperature (.about.80K) from 2D to 3D
phonon behavior. That is significantly better than the IR sensor
available today. By comparison, macroscopic freestanding SWCNT
networks described herein are expected to show sensitivity to 0.12
.mu.W with sample of 3.5.times.0.5 mm, which should give an
improvement by approximately one order of magnitude (6.85
.mu.W/cm.sup.2).
[0022] By way of further example, single-walled carbon nanotubes
(SWCNT) are expected to provide very desirable IR detection
properties when incorporated into the apparatus described herein.
The inventors continue to investigating various freestanding
single-walled carbon nanotube (SWCNT) networks for the detection of
mid- and far-IR radiation. Suspended SWCNT ribbons exhibit
extremely large bolometric response over a wide temperature range,
including room temperature. Thus, SWCNT network is an ideal
candidate for developing a low cost uncooled IR sensor. Recently,
devices based on individual tubes have shown promising results not
only in photodetection but also in multiple sensor applications
(e.g. chemical, biological agents). However, they are extremely
difficult to mass-produce reliably, because each individual
nanotube is different. Scaled-up production of such devices can be
impossible. The inventors approached this long-felt need by
employing a uniform, sparse network of SWCNT, which is then
electrically contacted, characterized and under-etched to form an
active freestanding, conductor-suspended SWCNT region with high
bolometric response. Statistical response of such a freestanding
suspended SWCNT network should be comparable to that of individual
nanotubes that exhibit very high absorption coefficient, but will
be more consistent, predictable and reliable. Moreover, a
significant advantage is that the methods herein will permit
production of large array of such active devices (pixels) on a
single substrate with minimal fabrication steps as compared to
known methods, thereby providing an efficient focal plane array
(FPA). Such FPAs should be able to operate at room temperature with
comparable or better sensitivity than the current vanadium oxide
based FPAs.
[0023] In one embodiment, provided herein is an uncooled infrared
(IR) sensor based on single-walled carbon nano tubes (SWCNT) in a
novel design. The key feature of the device is a sparse network of
freestanding SWCNT film as the IR sensing element, which is
connected to electrical resistance measuring component via metal
pads. Primarily due to extremely small thermal mass and other
unusual properties of SWCNTs, and further due to fewer and simpler
processing steps in fabricating a focal plane array (FPA), the
proposed device is expected to provide significantly superior
performance and lower cost compared to current state-of-the-art
uncooled IR FPAs. The key advantages of the proposed IR sensor are
summarized in Table 1 below.
TABLE-US-00001 TABLE 1 Performance Criterion Improvements over
current technology 1 Sensitivity (R* C.sub.TCR) ++ (significant
improvement) 2 Detectivity D* (1/G) ~ (comparable or better) 3 Time
constant .tau. (C/G) +++ (dramatic improvement) 4 Manufacturability
and Cost + (significant improvement) Key = R: electrical resistance
of each pixel. C.sub.TCR: thermal coefficient of electrical
resistance. G: thermal conductance of each pixel and associated
structure. C: heat capacity.
[0024] Evaluation of the Key Characteristics of the Sensors. The
inventors have provided an evaluation of an embodiment of the
sensor employing single walled carbon nanotube as an IR Sensor. The
evaluation of the key parameters pertaining to the performance of
the proposed single walled carbon nano tube (SWCNT) uncooled IR
sensors is summarized below.
[0025] Sensitivity. The sensitivity of a resistive bolometer in
measuring a temperature change, .DELTA.T, is given by its
temperature coefficient of resistance, C.sub.TCR. The sensitivity
or change in electrical resistance due to a unit change in
temperature is given by:
.DELTA.R/.DELTA.T.about.R.sub.T.sub.0C.sub.TCR
where R.sub.T0 is the base resistance. A high value of C.sub.TCR,
therefore, implies high sensitivity. At present for the
state-of-the-art uncooled bolometers the materials of choice are
vanadium dioxide (VO.sub.2), and amorphous Si. Vanadium dioxide
based materials exhibit fairly high TCR, typically 0.02 to 0.025/K.
For the a-Si based devices, TCR values in the range 0.028 to
0.043/K have been reported as seen in Table 2. Comparisons of TCR
at 350K versus length for a SWCNT, MWCNT and copper wires are
available. Although higher TCR values can be obtained with VO.sub.x
(x>2), it is not preferred since the reproducibility of such
films is problematic.
TABLE-US-00002 TABLE 2 The results of a recent study of a-Si based
thermo-sensing layers; comparison of the characteristics of
micro-bolometers. Pixel Pixel Voltage Thermo- E.sub.a TCR, .alpha.
area, A.sub.b resistance, responsivity, sensing layer (eV)
(K.sup.-1) (.mu.m.sup.2) R.sub.b (.OMEGA.) R.sub.U (V W.sup.-1)
a-Si:H,B 0.22 0.028 48 .times. 48 3 .times. 10.sup.7 10.sup.6
a-Si.sub.xGe.sub.y:H 0.34 0.043 70 .times. 66 1 .times. 10.sup.5 2
.times. 10.sup.5a a-Si.sub.xGe.sub.yB.sub.z:H 0.21 0.027 70 .times.
66 1 .times. 10.sup.6 2.8 .times. 10.sup.5a a-Si.sub.xGe.sub.y:H
0.34 0.043 70 .times. 66 5 .times. 10.sup.8 7.2 .times. 10.sup.5a
Current res- Spectral Thermo- ponsivity response Detectivity,
D.sup.a sensing layer R.sub.I (A/W) (.mu.m) (cm Hz.sup.1/2W.sup.-1)
a-Si:H,B -- 5-14 -- a-Si.sub.xGe.sub.y:H 0.3-14 2-14 4 .times.
10.sup.9 .+-. 1 .times. 10.sup.9 Sandwich structure a- 2.6 .times.
10.sup.-2 2-14 5.9 .times. 10.sup.9 .+-. 3.6 .times. 10.sup.8
Planar Si.sub.xGe.sub.yB.sub.z:H structure a-Si.sub.xGe.sub.y:H 2
.times. 10.sup.-3 2-14 7 .times. 10.sup.9 .+-. 3.3 .times. 10.sup.8
Planar structure .sup.aVoltage responsivity R.sub.U, was calculated
from the current responsivity R.sub.I.
[0026] The TCR for SWCNT reported by different authors varies
greatly presumably due to the presence of unknown percentages of
metallic and semiconducting tubes. Recently, theoretical estimates
were made using a bundle of tubes with graphene-like properties,
giving TCR=0.005-0.007/K at 350K, depending on length, as shown on
FIG. 1. However, Itkis et al. in a recent Science article reported
TCR values between 1 and 2.5% (0.01-0.025/K) for as grown SWNT film
in the 330 to 100 K temperature range. These values are comparable
to that of vanadium dioxide. If metallic tubes are selectively
eliminated from the SWCNT network, for example by preferential
Joule heating, the TCR of remaining semiconducting network will be
significantly enhanced. Furthermore, if we dope SWNCT with Si, the
bandgap and, hence TCR, can be further increased in a controlled
manner. Additionally, chemical functionalization of the network can
enhance desirable properties and eliminate undesirable properties
depending upon the desired use and performance of the network and
apparatus.
[0027] The inventors believe that the TCR of SWCNT can be increased
also by reducing the film thickness, modifying the processing
conditions, and by chemical functionalization of the tubes whereby
we modify the inter-tube contacts. For example, depending on the
nature of functionalized layer, the probability of tunneling, and
thus the value of TCR may double for a 10% change in energy
barrier. Therefore, the inventors expect that the TCR values would
be at least as good as that of currently used VO.sub.2, but
potentially several times higher with the predominantly
semiconducting network. Further, the R.sub.T0 of SWCNT can be
significantly higher than that of VO.sub.x, giving rise to
additional improvements in the sensitivity. It was shown that
relatively simple room temperature chemical modifications of
network can change its conductance by 2 orders. Thus, while the
existing SWCNTs have shown TCR comparable to that of VO.sub.x,
further improvements can be realized for the reasons outlined
above. We anticipate conservatively that x2 improvement can be
achieved in the sensitivity.
[0028] Detectivity, D*. In the limit of temperature fluctuation
noise, the D* of a thermal sensor is inversely proportional to the
square root of G where G is the thermal conductance. For the
radiation limit, assuming sensor can radiate only on one side, the
theoretical maximum for G .about.4.times.10.sup.-9 W/K. This yields
a maximum D* at room temperature of .about.2*10.sup.10 cm
Hz.sup.1/2 W.sup.-1 compared to currently achievable values in the
range .about.(4-7).times.10.sup.9 cm Hz.sup.1/2 W.sup.-1. G is the
total thermal conductance between the bolometer and its surrounding
consisting of several components. Heat may dissipate to the
surroundings through the SWCNT base legs or gas atmosphere, by
radiation, and by gas convection. In most practical cases, the
thermal conduction through the bolometer legs will dominate the
overall loss. For a VO.sub.X bolometer operated in a vacuum, G is
estimated to be .about.3.7.times.10.sup.-8 W/K. Although SWCNTs are
predicted to have high thermal conductance, we expect that in an
FPA configuration the thermal conductance of the sensor will be
determined by the relatively low value of the pads. Assuming that
the thermal isolation legs between pixels would be similar to those
designed for VO.sub.x bolometers, the G value of the SWCNT would be
comparable. As a result, D* values of SWCNT bolometer FPA would be
comparable or higher (owing to a higher TCR as noted in Sec 1) than
that of VO.sub.X FPAs.
[0029] Thermal time constant (.tau.). One of the issues with a
thermal bolometer is the residual memory between frames. It is
important that the heat from a scene imaged onto the pixel in one
frame must essentially conduct away before the heat in the next
frame can be detected. Otherwise, residual memory from the previous
frame smears the image if the scene changes. The thermal time
constant of a bolometer, given by C/G where C is thermal capacity,
determines the residual memory effect. In general, .tau. should be
about 1/3 the frame time such that the memory effect is minimal.
For VO.sub.x C is .about.4.34.times.10.sup.-10 J/K and for G
.about.3.7.times.10.sup.-8 W/K, .tau.=12 msec. Using the 1/3 rule,
this gives .about.30 Hz frame rate typical of most commercial
bolometer FPAs. The volume and mass (thus heat capacity) of the
SWCNT absorber are a few orders of magnitude smaller than the
current VOx thin film devices. Assuming worst-case-scenario we
estimate a value of 8.4.times.10.sup.-14 J/K for SWCNT, which is
lower by .about.4 orders of magnitude compared to VOx. For the same
G, we would then expect a time constant in the order of
microseconds, resulting in a frame rate of the order of MHz! Under
practical conditions, we may expect frame rates of several to 100's
of kHz with SWCNT bolometers. This orders of magnitude improvement
in fast imaging of the infrared scene would greatly expand the
application domain of the proposed bolometers, especially for
situational awareness that require high temporal resolution.
[0030] Manufacturing and cost. The fabrication of current VO.sub.x
based two-level FPAs requires as many as 15-25 steps See R. R.
Neli, I. Doi, J. A. Diniz, and J. W. Swart. Development of process
for far infrared sensor fabrication. SENSORS AND ACTUATORS
A-PHYSICAL, 132, 400-406 (2006) and S. Franssila. Introduction to
Microfabrication. WILEY-V C H VERLAG GMBH, 2004, involving
sacrificial layers, complex film deposition, lithography, and other
costly processes. By comparison, our exploratory work indicates
that the proposed fabrication of SWCNT bolometers is very simple.
It would include growing SWCNT network, depositing readout contacts
and etching out oxide substrate. Thus the manufacturing of the new
devices is expected to be much less complex, with better
reproducibility, resulting in substantial cost advantages as well.
The ability to fabricate finer features in SWCNT than in VO.sub.x
films would allow smaller pixel size or higher pixel density in the
former material, resulting in higher image resolution.
[0031] Summary of expected benefit of SWCNT bolometers. SWCNT based
bolometers are expected to have sensitivity and detectivity
comparable but potentially higher than with the current VO.sub.x
based FPAs. SWCNT based bolometer FPAs would enable dramatically
higher frame rate (.about.kHz) operation, which would be suitable
for fast imaging applications requiring high temporal resolution.
SWCNT based bolometers are expected to be less expensive since our
process has fewer lithographic steps and is much less complex.
Finally, since we can obtain uniform sparse network across the
whole substrate, pixel size can be scaled down to increase the
total number of pixels as well as pixel density significantly.
[0032] While this description is made with reference to exemplary
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope. In addition,
many modifications may be made to adapt a particular situation or
material to the teachings hereof without departing from the
essential scope. Also, in the drawings and the description, there
have been disclosed exemplary embodiments and, although specific
terms may have been employed, they are unless otherwise stated used
in a generic and descriptive sense only and not for purposes of
limitation, the scope of the claims therefore not being so limited.
Moreover, one skilled in the art will appreciate that certain steps
of the methods discussed herein may be sequenced in alternative
order or steps may be combined. Therefore, it is intended that the
appended claims not be limited to the particular embodiment
disclosed herein.
* * * * *